Recent years have seen increasing use of multimedia services in which various services relating to information transmission or information provision are provided by use of radio. Numerous wireless terminals have been developed to this end and put into practical use. The applications of these services have become increasingly diversified to include telephones, televisions, LANs (Local Area Networks), and so on. To enjoy all of these services, the user is required to own the wireless terminal corresponding to the respective services.
With a view to achieving greater convenience for the user to enjoy these services, moves have been started to provide multimedia services to the user whenever and wherever without making the user aware of the presence of the media, that is, in a ubiquitous manner. In this regard, a so-called multi-mode terminal, which realizes the provision of plural information transmission services by means of a single terminal, has been partially realized.
An ordinary ubiquitous information transmission service by radio uses electromagnetic waves as the transmission medium. Accordingly, within the same service area, one frequency is used for one kind of service, thereby allowing provision of plural kinds of services to the user. A multimedia terminal has thus the function of transmitting and receiving electromagnetic waves of plural frequencies.
According to a method adopted in conventional multimedia terminals, for example, plural single-mode antennas each corresponding to one frequency are prepared, and these antennas are mounted to a single wireless terminal. With this method, in order to operate the respective single-mode antennas independently, these antennas must be mounted while being spaced from each other at a distance substantially corresponding to the wavelength. In this regard, the frequencies of the electromagnetic waves used for typical ubiquitous information transmission services are limited to the range of several hundred MHz to several GHz due to the restrictions associated with the free space propagation phenomena, so the distance between neighboring antennas becomes several tens of cm to several m. The dimensions of the terminal thus become large, which detracts from the convenience in terms of portability for the user. Further, due to the spaced arrangement of the antennas having sensitivities to different frequencies, the RF circuits to be connected to the antennas must be also installed while being separated from each other in correspondence with each of these frequencies.
It is thus difficult to employ the semiconductor integrated circuit technique, which may result not only in an increase in the dimension of the terminal but also an increase in the cost of the RF circuit. When the integrated circuit technique is forcefully applied to achieve higher integration of the overall circuit, there arises the need to provide the connection between the RF circuit and each individual antenna placed at a distance therefrom by means of an RF cable. Incidentally, the radius of the RF cable applicable to a terminal of a size that can be carried by the user is in the vicinity of 1 mm. Accordingly, currently, the transmission loss of the RF cable reaches several dB/m. Due to the use of such an RF cable, the electric power consumed by the RF circuit increases, which causes a marked reduction in the time for which the terminal providing ubiquitous information services can be used or a marked increase in the terminal weight due to the increased battery volume, thus seriously impairing the convenience of the user using the terminal.
One of the important elements for overcoming these problems associated with the multi-mode wireless terminal that provides multiple information services to the user is a multi-mode antenna having sensitivity to electromagnetic waves of plural frequencies. There have been proposed several multi-mode antennas having a single antenna structure and a single feeding point meeting to plural frequencies, in which electrical connection with an RF circuit portion of the multi-mode terminal is made to enable transmission/reception of a communication signal between the free space and the RF circuit portion.
Examples of conventional multi-mode antennas include, for example, the two-mode antenna disclosed in Japanese Patent Laid-Open No. 2003-101326 (Document 1). In this antenna, a part of a conducting plate is removed to form a U-shaped slit, with an L-shaped conductor being added into the U-shaped slit. The U-shaped slit operates at a first frequency, and mainly the L-shaped conductor operates at a second frequency. The electromagnetic radiation mechanism in each frequency range is accomplished by mutually perpendicular radiation elements including respective structures.
As another example of a conventional two-mode antenna, Japanese Patent Laid-Open No. 2003-15243 (Document 2) describes an antenna having two opposing linear conductors formed in the inner portion of a conductor having a slit. Each linear conductor also operates as a feeding line of the slit, and transmission/reception of electromagnetic waves of different frequencies is performed between the slit and the feeding line. The principle of operation is the same as that of Document 1.